1. Field of the Invention
The present invention relates to a gateway device which mutually connects, for example, between a public switched telephone network and an Internet Protocol (IP) network, and a communication system provided with this kind of device.
2. Description of the Related Art
As needs for information communication become high and liberalization of communication advances, information communication services including voice and data communication have diversified. In accordance with such a background, the number of carriers who newly enter a communication service field has increased, and service competition among the carriers has become serious. The new carriers are called new common carriers (NCCs), and provide a variety of services by using techniques such as voice over Internet Protocol (VoIP), etc. VoIP is a technique which integrates a voice system network and a data system network by packetizing to transmit digital voice data.
The NCCs often lease facilities such as exchanges from a specific carrier which owns subscriber lines at predetermined charges. For the most part of the NCCs construct their own networks such as IP networks by their own funds. In addition to these networks, by adding a public switched telephone network (PSTN) of a specific carrier, a communication system is formed. To provide services to subscribers (users), these facilities are utilized in an interdisciplinary manner.
For mutually connecting different communication networks like the PSTN and the IP network, a gateway device is utilized. This kind of device includes an IP conversion unit to convert voice data and binary data into IP packets, and a packet switching unit which switches the IP packets. All of these units act as network interfaces for the IP network.
Some of this kind of gateway devices work with base stations of a mobile phone system. The mobile phone system includes a so-called a cellular phone system and, in Japan, a so-called personal handy-phone system (PHS). Hereinafter, this kind of a radio communication system is generically called a mobile communication system.